1. Field of the Invention
The present invention relates to a device for detecting a three-dimensional position/orientation of an object such as one of workpieces stacked randomly, which is suitable for use in an apparatus for picking up an object using a robot.
2. Description of Related Art
As a method of detecting a three-dimensional position/orientation of an object such as a workpiece on the basis of a picture of the object taken by a video camera, there is know a method called a stereo mode in which a plurality of video cameras are used. In some cases, three or more video cameras are used, however, those cases can be reduced to a stereo mode using two video cameras, by focusing on two of the video cameras. Hence, the description below will be limited to the case in which only two video cameras are used.
What is difficult in detecting an object in the stereo mode is so-called “determination of corresponding images”, namely determining which part of a picture taken by a video camera corresponds to an image of a particular part of an object detected in a picture taken by another video camera. An example of determination of corresponding images is seen in, for example, Japanese Patent No. 3235364. In this Patent, determination of corresponding images is performed, limiting to those objects which have a contour including a circular or circular-arc part, and depending on the fact that pictures are locally similar, and the fact that characteristic points are distributed in a circular arc.
Like this, generally in the stereo mode, an attempt to identify corresponding images is made on a local basis. However, since it is based on the local comparison between pictures, accuracy and reliability tends to be unsatisfactory. This is because it is expected that in not a few cases, the probability of the local comparison of pictures finding a lot of similar parts will not be low.
There is a technique which takes geometrical restrictions obtained from arrangement of two video cameras (epipolar restriction, etc.) into consideration to thereby improve the reliability of the determination of corresponding images. However, the technique is not adequate in many cases. Needless to say, error in determination of corresponding images directly leads to error in detection of the position of an object. In the above-mentioned Japanese Patent No. 3235364, reliability is improved by defining the shape of an object to be detected more specifically and thereby increasing restrictions in the determination of corresponding images.
However, as a matter of course, this technique has a drawback that it narrows the range of detectable objects.
Further, the conventional method of detecting a three-dimensional position/orientation of an object in the stereo mode has a problem that if objects overlap, so that part of an object is hidden behind another object, determination of corresponding images cannot be performed on the partly hidden object.